1. Field of the Invention
The present invention relates to a communication device provided with a bus having a signal line supplied with a pull-up voltage, and for transmitting a predetermined signal.
2. Description of the Related Art
Communication devices provided with a bus having a signal line supplied with a pull-up voltage, and for transmitting a predetermined signal are well known.
For example, JP-A-2005-18249 (hereinafter referred to as Patent Document 1) discloses that by providing an interface circuit with a switch for switching between a first power supply and a second power supply, a plurality of pull-up resistors connected to the switch, and a plurality of data bus lines connected respectively to the plurality of pull-up resistors, in the case in which the mother board is changed, the power supply voltage of the interface circuit can easily be changed in accordance with the power supply voltage of the mother board without changing the pull-up connection of the interface circuit.
Further, JP-A-Hei 6 (1994)-348378 (hereinafter referred to as Patent Document 2) discloses a register unused bit processing circuit which can be reduced in circuit scale by commonly using one new bus driver instead of a plurality of bus drivers for unused bits.
Further, Japanese Utility Model Registration No. 3096674 (hereinafter referred to as Patent Document 3) discloses a bus communication device which lowers the signal level from 5V to 3.3V in the intermediate portion of the bus when sending an H-level signal of 5V from a microcomputer to a device, and then input the signal to the device after restoring the signal to the H-level signal of 5V, thereby preventing the noise generated in the Inter-Integrated Circuit (IIC) bus from influencing analog signals, thus downsizing the circuit board.
Further, Japanese Utility Model Registration No. 3120526 and Japanese Utility Model Registration No. 3110950 (hereinafter referred to as Patent Documents 4, 5) disclose a television set having various circuits such as a microcomputer or a tuner circuit connected to each other via a bus.
Here, the IIC bus described above is a two-wire serial bus composed of a clock line and a data line, and is described as “I2C bus” in some cases. FIG. 5 is a block diagram showing an example in which the IIC bus connects between various kinds of devices such as a microcomputer provided to a television set. In FIG. 5, a main microcomputer 2 and an EEPROM 3, which is a kind of a nonvolatile semiconductor memory, are devices (hereinafter referred to as 3.3V system devices) which are driven by a power supply (hereinafter referred to as a 3.3V system power supply) for supplying a voltage of 3.3V during a standby state and a power-ON state, and can be driven with the voltage (hereinafter referred to as a pull-up voltage) for pulling up the signal line of the I2C bus 1, in a range of 3.3V through 5V. Further, a tuner (TUNER) 4 and a circuit 5 controlled via the I2C bus 1 are devices (hereinafter referred to as 5V system devices), which are driven by a power supply (hereinafter referred to as a 5V system power supply) for supplying a voltage of 5V during only the power-ON state, and can be driven with the pull-up voltage of 5V. Further, in the I2C bus 1, the solid line shows a line for performing communication during both the standby state and the power-ON state, and the broken line shows a line for performing communication during only the power-ON state. It should be noted that the standby state described above denotes, for example, a standby state in which a main power is in an ON state, only a few circuits such as the microcomputer are supplied with the power waiting for various operations (e.g., a power-ON operation) by the user, or waiting along a reservation program. Further, the power-ON state described above denotes, for example, a normal operation condition in which other circuits necessary for the operation are also supplied with the power for displaying picture on a screen or outputting sounds from a speaker.
In order for performing communication including the broken line sections during the power-ON state in the configuration including a signal line of the bus having different communication paths between the standby state and the power-ON state as described above, since the 3.3V system power supply is not capable of supplying the pull-up voltage, it can be considered to operate the 5V system power supply during also the standby state to supply the pull-up voltage by the 5V system power supply. In such a case, the 5V system devices are also driven during the standby state, thus there is a possibility that the power consumption (so-called standby power) during the standby state does not comply with the requirements of the International Energy Star Program.
Therefore, it can be considered that as shown in FIG. 6, to newly add a bus-dedicated power supply (hereinafter referred to as I2C-dedicated power supply) 6 for only supplying the 5V pull-up voltage. Alternatively, as shown in FIG. 7, it can also be considered that the pull-up voltage between the 3.3V system devices is supplied by the 3.3V system power supply while the pull-up voltage between the 5V system devices is supplied by the 5V system power supply, and at the same time, a pull-up voltage conversion circuit (hereinafter referred to as a 3.3V←→5V conversion circuit) 7 having a MOS configuration, for example, is newly added in the signal line of the I2C bus.
However, there is a possibility that increase in cost is caused by newly adding the I2C-dedicated power supply or the 3.3V←→5V conversion circuit 7. Further, in the case of FIG. 6, there is a possibility that increase in the standby power is caused by raising the pull-up voltage to 5V.